The present invention relates to a color display device employing a color cathode ray tube, and in particular to a color display device capable of suppressing occurrence of moire while retaining good focus characteristics of a color cathode ray tube.
As color cathode ray tubes for use in color display devices such as a color TV receiver and a color display monitor for a terminal of office automation equipment, color cathode ray tubes are widely used which utilizes a shadow mask as its color selection electrode. The shadow mask type color cathode ray tube includes a vacuum envelope comprising a generally rectangular panel portion, a generally cylindrical neck portion and a funnel portion for connecting the panel portion and the neck portion together, a phosphor screen containing a large number of dot-shaped or stripe-shaped phosphor picture elements and formed on the inner surface of the panel portion, an electron gun housed within the neck portion for projecting three in-line electron beams, and a shadow mask fixed to a mask frame in a closely spaced relationship from the phosphor screen within the vacuum envelope, and having a large number of electron beam-transmissive apertures so as to serve as a color selection electrode.
The shadow mask type color cathode ray tube is equipped with a deflection device comprising horizontal and vertical deflection coils combined with a ferromagnetic yoke and mounted around the transition region between the neck portion and the funnel portion, is also equipped with a magnet assembly around the neck portion for color purity correction and beam centering. The color display device is made up of the shadow mask type color cathode ray tube, a deflection circuit for driving the deflection device and a circuit for driving the color cathode ray tube.
The electron beams projected from the electron gun are deflected in horizontal and vertical directions by the deflection device, then pass through electron beam-transmissive apertures in the shadow mask serving as the color selection electrode and then impinge upon the intended phosphor picture elements to form an image on the phosphor screen.
The above-explained color display device has a disadvantage that the so-called moire occurs at the right and left sides of the phosphor screen of the color cathode ray tube during its operation and it degrades the quality of a displayed image. Such moire is pronounced especially in the high-definition color display device.
Moire is caused by a periodic appearance of alternate light and dark portions produced depending upon positional relationship between the electron beams and the phosphor picture elements, and is perceived as stripes on the phosphor screen and therefore degrades the quality of the display. It is known that, the smaller the vertical diameter of the electron beam spot, that is, the minor axis of the electron beam spot is vertical, the more pronounced the moire.
FIG. 5 is a front view of the panel portion of the color cathode ray tube of the color display device, and shows the shapes of electron beam spots at various positions on the phosphor screen when an image is displayed on the phosphor screen. In FIG. 5, the phosphor screen 2 of the panel portion 1 is divided into three regions (zones) of a central region (hereinafter a central zone) 3, left and right regions (hereinafter left and right zones) 4,5. Respective zones will be represented by clock positions.
The center beam spot 6 at the center of the central zone 3 has a major axis in the vertical direction and a somewhat vertically elongated shape, the top and bottom spots 7, 8 at the 12 and 6 o""clock positions are more elongated in the vertical direction, but the right and left spots 10, 9 at the 3 and 9 o""clock positions are horizontally elongated, that is, they have the major axes in the horizontal direction.
As is apparent from FIG. 5, the right and left electron beam spots 10, 9 at the 3 o""clock and 9 o""clock positions have vertical diameters smaller than those of the electron beams in the remaining positions, and consequently, the moire is pronounced in the areas corresponding to the right and left zones 5 and 4.
A conventional measure against such moire is such that the entire raster is moved alternately upward and downward on successive fields by moving the electron beam path a small distance alternately upward and downward on successive fields such that light portions of one field are superimposed on dark portions of the next field, and vice versa, and consequently, moire is not visually perceived.
A concrete method for the above conventional measure is to superimpose a moire-correcting rectangular current on vertical deflection current flowing through the vertical deflection coil of the deflection device on every alternate field so as to move the entire raster alternately upward and downward.
The above conventional method is expected to provide some effects for eliminating moire compared with the case where such a method is not utilized. However, this method moves the raster alternately upward and downward parallel with the vertical sides of the raster on successive fields to make the moire imperceptible to the eye, and consequently, this method deteriorates focus characteristics at the central zone which is the most important area for a display, and this is a fatal problem other than moire, and therefore there is a demand for a further measure against moire in the color display device.
It is an object of the present invention to provide a superior color having good display characteristics by solving the above problems.
The following describes a representative configuration of the present invention for achieving the above object.
In accordance with an embodiment of the present invention, there is provided a color display device comprising: a color cathode ray tube including a phosphor screen, a color selection electrode closely spaced from the phosphor screen and an electron gun for projecting plural in-line electron beams toward the phosphor screen; a deflection device mounted around the color cathode ray tube and including a horizontal deflection coil and a vertical deflection coil for deflecting the plural in-line electron beams horizontally and vertically, respectively; an electron beam correction apparatus mounted around the color cathode ray tube and including a correction coil wound around a tube axis of the color cathode ray tube; a deflection circuit for driving the horizontal and vertical deflection coils; and an electron beam correction circuit for supplying to the electron beam correction apparatus a generally rectangular-wave signal having a period equal to two times a period of the vertical deflection of the plural in-line electron beams and in synchronism with the vertical deflection of the plural in-line electron beams.